1. Field of the Invention
The embodiments of the invention generally relate to metal-gate high-K integrated circuit structures and, more particularly, to an integrated circuit structure with same conductivity type transistors having different high-k-metal gate structures in order to achieve different threshold voltages.
2. Description of the Related Art
As complementary metal oxide semiconductor (CMOS) devices are scaled in size, conventional gate stack structures are being replaced by high-k dielectric-metal gate stack structures. Specifically, a conventional gate stack structure typically includes a thin silicon oxide (SiO2) gate dielectric layer and a doped-polysilicon gate conductor layer. Unfortunately, doped polysilicon gate conductor layers are subject to depletion effects. These depletion effects result in an increase in the effective gate dielectric layer thickness and, thereby limit device scaling. Thus, high k dielectric-metal gate stacks with different work functions for n-type field effect transistors (NFETs) and p-type field effect transistors (PFETs) have been introduced. These stacks are improvements over the conventional gate structures in that the high k-dielectric layer minimizes leakage current and the metal gate conductor layer is not subject to depletion effects. Additionally, these high-k dielectric-metal gate stack structures offer new opportunities for constructing improved analog functions.
In conventional gate stacks, the gate electrodes comprise degenerately doped polysilicon, with p-type doping employed to set gate Fermi levels at the silicon valence band and n-type doping employed to set gate Fermi levels at the silicon conduction band. FETs employing such p-type and n-type gates that are otherwise identical, will have threshold voltages differing from one another by very nearly the silicon band-gap voltage, (i.e., approximately 1.1 Volt). High-k dielectric-metal gate stacks can have differing effective work functions by design by employing various charge levels in the gate-stack dielectrics and by choosing or adjusting gate electrode materials to adjust electrode work functions. Thus pairs FETs employing differing high k-dielectric-metal gate stack structures, but otherwise being identical, can have selectable differences in threshold voltage, which are determined by differences in gate stack effective work functions.